In 2003, at McGraw-Hill, we discovered something about making ebooks while working with Dr. Bill Detmer and Unbound Medicine to create Harrison’s On Hand. Don’t start or present through the Table of Contents; start with the Index.
Ten years later and the lesson’s being rediscovered. Read Hugh McGuire’s A Publisher’s Job Is to Provide a Good API for Books – Tools of Change for Publishing.
Books On Books 
Nicely said! And if you need a hand, the American Society for Indexing (www.asindexing.org) is always available.
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